Author Topic: Physics Problems with Large Wooden Clock  (Read 9435 times)

brucegregory

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Physics Problems with Large Wooden Clock
« on: October 27, 2007, 07:42:44 AM »
Hello all of you extraordinarily patient woodworkers, (you must be so, or you would not be working on wooden clocks).

Having successfully made 4 scroll saw clocks from plans - that actually worked, (others did not), I decided I was ready for the big league, that is, taking on a "clock commission" from a local doctor for some thousands of dollars.  Taking money in advance, (my first mistake), I have spent the last eight months trying to get an original clock design of mine, a very large one, to work.  I did get it to run for about 8 hours on 18 pounds of weight, but after trying to refine it a bit, I cannot keep it running anymore.

I used ball bearings for all of my pivots to insure a lack of friction in the most obvious places, but friction, or inertia must be coming into play in other areas of the design.

My instinct tells me that because of the large mass in all of the gears due to their size and thickness, (the great wheel is 14 inches in diameter and 3/4" thick - made of hard maple and red oak), more weight than that required for smaller designs would be necessary in order to overcome the initial inertia of all the gears in the train.  18 pounds seems somewhat excessive, however, though I think the overall construction heft can bear it.

After many hours of observation, I noticed that most of the friction seemed to be centered around the escape wheel and pallet.  I made them of much smaller diameter and thinner than any of the other wheels in the train to lessen the final mass - 3/8" thick by 6 inches in diameter for the escape wheel.  At first, when both pallet and wheel were in total contact, the clock would not run, but, after moving the pallet so that only a small portion of its surface was in contact with the wheel, it ran for 8 hours straight.  I figured that, maybe, by reducing the contact area of the pallet teeth, by making them thinner, I would produce the desired effect.  The actual result of the change was that the pallet began to bounce a little with each impact on the escape wheel - (I'm using a recoil escape design).  The bounce also introduced a "shudder" into the crutch and then into the pendulum, thus wasting energy.  Thinking that I had reduced the mass of the pallet too much by thinning the teeth, I added weight to each side of it, equally, until the bounce was minimized.  The clock only runs for a few minutes, now.

I'm at a loss as to how I should proceed to troubleshoot the design.  I'm pretty sure all of the gearing is as near perfect as it can be, since I cut everything with my CNC mill.  I used a pie shape sectional design for each gear, both to avoid the "plywood" effect and to allow the grain of the wood to be aligned parallel to each tooth.  It also looks great.  The downside to this kind of construction is that the gears are fairly heavy, however, and maybe this is where the trouble begins and ends.

Eight months is too long to work on one clock.  I'm really worn out on this one, and the creative juices are beginning to ebb, somewhat.  Anybody have any suggestions as to how I should go forward, (or backward)?

Thanks for listening,

Greg Smith
Flagstaff, Arizona

Offline bobledoux

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Re: Physics Problems with Large Wooden Clock
« Reply #1 on: October 28, 2007, 07:15:55 AM »
I'm a relative newcomer to this business, but I spent several months getting a large set of clock plans to work correctly.

I don't think the great wheel mass is contributing to the problem.  After all, it it has great inertia being driven by a large weight and it is a long ways "upwind" from the escape wheel.

Have you read Rabbit's views on clock bushings?  http://www.bealltool.com/clockforum/index.php?topic=25.0

Ball bearings, especially shielded bearings, have significant drag due to the shields.  They are not a good application for oscillating wheels, like a recoil escape wheel.  They are appropriate on a great and maybe center wheel, but as you get closer to the escape wheel, the bearing drag can exceed the driving force.

If you remove the pallets will the system run in a smooth manner?  If you repeatedly stop the escape wheel does it start back up in a consistent manner?

Hopefully, some of the more experienced builders will join in on your problem. 

Offline rabbit

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Re: Physics Problems with Large Wooden Clock
« Reply #2 on: October 28, 2007, 10:48:49 AM »
i agree; i don't think the mass/inertia is your problem. i've seen and built much larger.
and 18 pounds is certainly a lot. how far is the drop, and what is the run time?
i think it's either a friction issue or something wrong with the escape.
how free is the turning without the escape (with the pallet removed)? the whole train should turn very freely - only the slightest torque on the great wheel should be required. if not, check each pivot individually (no other gears installed). if that passes, check each gear mesh individually (two gears at a time). there may be either a tooth profile issue, or a depthing issue.
if all is well here, the escape may be the culprit. recoil escapes are very forgiving, but a wooden one can't easily be "adjusted" (you can't "bend" the pallets). re-check the geometry.
this sounds like a cool clock. good luck getting it fixed. let's see it.
- rabbit
- rabbit

brucegregory

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Re: Physics Problems with Large Wooden Clock
« Reply #3 on: October 28, 2007, 12:00:12 PM »
Dear Clock Friends:

I have checked the running of the clock without the pallets and it does run smoothly with a very little pressure applied downward on the Great Wheel. I was concerned about the slight amount of drag produced by closed bearings, but I don't think this is what is stopping the clock, but might contribute to the amount of weight necessary to drive it.

I did find a short tooth on the third wheel that is the culprit for stopping the clock.  I'm going to make this wheel over again.

One of the things that puts me in a bind is the client's absolute insistence that no plywood be used, self-made or otherwise.  So, I was forced to use a "pie-shaped" gear construction, which has its advantages and disadvantages.

Another thing I'm going to change is the diameter of the barrel.  I tried to make it as small as possible to allow for a greater running time - big mistake - as this eats up a lot of energy.  The drop of the weight is about 63", depending on how tall I make the "trunk" of the clock "tree".  The design is based on an organic, non-symmetrical tree shape, so, what I am testing right now are just the works as mounted between the tree branch plates.  The whole thing is securely mounted to a wall since the supporting "trunk" does not yet exist.

I also think I made both the escape wheel and the pallets too thick, and therefore they contain too much friction between them.  I tried experimenting by thinning the lifting planes of the pallets, but now there is a noticeable bounce whenever the pallets collide with the escape wheel, creating a corresponding shudder in the crutch and thence the pendulum shaft. I think I'll try making the pallets out of a denser wood and widen the lifting planes just a little.

It's strange, but after you have had your eyes and mind fixed on a project like this for so many hours, (nearly 1000 in this case), fatigue begins to set in and your powers of discouragement resistance get quite low.  Sometimes I am ready to start over completely or just take an axe to the whole thing.

Here is the address for some photos, (just parts, unassembled):  http://www.learnblender.com/WoodClock

By the way, I used CorelDraw to create everything.  It seems to be accurate enough to make segmented gears after saving out as .dxf and driving my CAM program.  Great program for drawing organic, tapering shapes, (like tree branches).

Nothing like being an indentured servant to yourself,

Thanks for the help,

Greg Smith
« Last Edit: October 28, 2007, 12:41:19 PM by brucegregory »

Offline bobledoux

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Re: Physics Problems with Large Wooden Clock
« Reply #4 on: October 28, 2007, 07:28:17 PM »
Impressive clock design.

You have more courage me.  I would never accept a commission for a clock I hadn't previously built.  There can be so many unexpected roadblocks to success.

The old Terry clocks were made with 8 day rewind cycles.  With the small wheels the inertia of the parts permitted  this.  As the clock parts get bigger the increasing mass seems to add new challenges to the design.  As the size increases I prefer to reduce the ratio between the great and center wheels.  This translates into more frequent rewinding but with better reliability.

Has anyone successfully built a 15 day clock, with large wood wheels, patterned after the regulator clock in the last chapter of Goodrich's book, "The Modern Clock?"  Here is one attempt, that failed until it was reduced to a 30 hour movement:

http://web.mac.com/johntribe/Clocks_-_John_Tribe/18_Goodrich_Regulator.html



Dave

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Re: Physics Problems with Large Wooden Clock
« Reply #5 on: October 30, 2007, 01:55:09 PM »
Hi Greg, a realy nice design and after so many hours work it should be running, its frustrating isnt it, but thats what happens when bringing something new into the world. I picked up on your saying that the pallets bounce, could that be a sign that as you have down sized the friction that you might have to do the same with the driving weight. There could be some interplay with the pallet angles and any free play in the crutch, there is certainly some material to come of the escape wheel tips. They need to be thinner but this will increase the drops so you might have to make a new set of pallets, also check the ballance of your escape wheel, with so much fancy fret work, it might not be true running, come back and let us all know how you get on, Dave